The present invention relates to a method for determining the radio refractive index ("RRI") structure of the atmosphere, and more particularly, to a method for determining the RRI structure of the atmosphere by detecting the strength of a radio frequency (RF) signal propagated over a predetermined path.
The radio refractive index of a medium such as the atmosphere is defined as n=c/v, where c is the speed of light in a vacuum and v is the speed of a radio wave in the medium. A typical value of n for air at the earth's surface is 1.000340. In the radio-wave propagation field, a dimensionless quantity, M, called the modified refractivity is defined as M=[n-1+h/a].times.10.sup.6, where h is the height above the surface of the earth and a is the radius of the earth. In the example given above, where n=1.000340 and h=0, M=340. However, it is not the numeric value of M which is important, but rather the change in M as a function of a change in altitude which determines the characteristic of the radio refractive index structure that bears on the prediction of radio wave propagation through the atmosphere. The relation of M as a function of height describes the vertical RRI structure of the atmosphere at a particular location.
The vertical characteristics of the radio refractive index structure of the atmosphere influence the propagation of radio waves over a large range of frequencies by causing the path of the radio waves to bend or refract as they pass through adjacent layers of the atmosphere. It is important to know the radio refractive index structure of the atmosphere in order to predict the performance of a wide variety of radio equipment. An example of a system that uses the radio refractive index structure for such purposes is given in U.S. Pat. No. 4,125,893 entitled "Integrated Refractive Effects Prediction System," incorporated herein by reference.
Present methods for determining the vertical structure of the radio refractive index generally require direct measurement of atmospheric properties at various altitudes. Examples of systems deployed to make such direct measurements include balloon-borne radiosondes that measure pressure, temperature, and humidity, and also include aircraft-mounted refractometers that measure the radio refractive index and altitude. However, all direct sensing methods have the disadvantages of being expensive, logistically complex, and incapable of providing a real time determination of the radio refractive index structure.
Referring now to FIG. 1, there is shown an example of a modified refractivity versus height profile 10, represented by a solid line. Data used to construct the modified refractivity profile 10, presented by way of example in FIG. 1, were derived from radiosonde measurements taken off the coast of the Point Loma area of San Diego, California. The profile 10 is ideally approximated by three linear and serially connected segments 12, 14, and 16, shown as dashed lines which together idealize the modified refractivity versus height profile 10 (the segment 14 and the profile 10 are substantially coincident).
Each of the segments 12, 14, and 16 represent successive layers of the atmosphere, where each layer is characterized as having a modified refractivity which varies linearly with altitude. For example, the layer 12 represents the modified refractivity of a layer of the atmosphere adjacent to the earth having a radio refractive index which increases linearly with increasing altitude up to an altitude referred to as the trapping layer base height. The segment 14 represents the modified refractivity of a second layer of the atmosphere which is coterminous with the first layer at the trapping layer base height. However, the modified refractivity of the second layer decreases linearly with increasing altitude starting from the maximum modified refractivity of the first layer. The segment 16 represents the modified refractivity of a third layer of the atmosphere which increases linearly with increasing altitude starting from the minimum modified refractivity of the second layer.
Five modified refractivity parameters are used to construct the three segments 12, 14, and 16 which represent the RRI structure of the atmosphere. These refractivity parameters are: 1) an M-unit versus height gradient (slope) of the lowest segment 12; 2) an M-unit versus height gradient of the highest segment 16; 3) a total M-unit excursion, where the M-unit excursion represents the difference of M between the maximum and minimum altitude, M.sub.14 of the second segment 14; 4) a vertical thickness, h.sub.14, of the second segment 14; 5) and a height z of the base of the second segment 14, which is usually the height of the base of a temperature inversion layer, also referred to as the trapping layer base height. Of the five parameters, the trapping layer base height is the dominant factor for influencing many propagation effects. The radio refractive index structure for the San Diego, CA example is shown in FIG. 1 to have a trapping layer base layer height of about 620 meters.
Thus, it may be appreciated that a need exists for a system and method for readily determining the radio refractive index structure of the atmosphere, and more particularly, for determining the base height of a temperature inversion layer in real-rime.